1. Field of the Invention
The present invention relates to an apparatus and a method for the rapid thermal processing (RTP) of sensitive electronic materials. The present invention reduces the thermal inhomogeneities introduced when the materials have inhomogeneous structure and optical absorption characteristics.
2. Description of the Prior Art
Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as a general, well controlled, method for heating objects or wafers which are in the form of thin sheets, slabs, or disks. The objects are inserted into a chamber which has at least some portions of the chamber walls transparent to transmit radiation from powerful heating lamps. The transparent portion of the walls is generally quartz, which will transmit radiation up to a wavelength of 3 to 4 microns. These lamps are generally tungsten-halogen lamps, but arc lamps or any other source of visible and/or near infra-red radiation may be used. The radiation from the lamps is directed through the transparent portions of the walls on to the surface of the object to be heated. As long as the objects absorb light in the near infrared or visible spectral region transmitted by the transparent portion of the walls, RTP techniques allow fast changes in the temperature and process gas for the different material processes and conditions. RTP allows the "thermal budgets" of the various semiconductor processing to be reduced, as well as allows the production of various metastable states which can be "frozen in" when the material is cooled rapidly.
RTP systems are relatively new. In the last 10 or 15 years, such systems were used only in research and development. The thrust of the work was increasing the temperature uniformity, and developing heating cycles and processes which decreased the thermal budget. Prior art RTP machines can heat unstructured, homogeneous materials in the form of a flat plate, and produce temperature uniformities across the plate adequate for semiconductor processing processes. However, when the material is not uniform and has, for example, non-uniform optical or material characteristics, relatively large temperature non-uniformities can occur. This is particularly injurious for a partially processed silicon wafer with various parts of devices implanted, etched, or grown on the wafer.
The temperature control in current RTP systems is mostly performed by monochromatic (or narrow wavelength band) pyrometry measuring temperature of the relatively unstructured and featureless backside of semiconductor wafers. The results of the temperature measurement are used in a feedback control to control the heating lamp power. Backside coated wafers with varying emissivity can not be used in this way, however, and the backside layers were normally etched away or the temperature was measured using contact thermocouples.
A newer method of temperature control is the power controlled open loop heating described in our U.S. Pat. No. 5,359,693, which patent is hereby incorporated by reference. This patent also discloses a method of producing relatively defect free material in RTP machines. The number integrated circuits using sub-half-micron technology has increased enormously since we filed for our patent, and the methods outlined are not sufficient to produce the necessary material quality. The structure dependent thermal and process inhomogeneities must be reduced to the absolute minimum physically possible.
Apparatus induced thermal inhomogeneities have been reduced in the last few years because of the demand for more uniform processing. Among the techniques used have been control of the individual lamp power, use of circular lamps, and rotation of the semiconductor wafers with independent power control.
The reduction of temperature non-uniformities due to non uniformities and structures on the material being processed, however, presents many more problems. Thin films on the surface of the heated structure can have constructive or destructive interference effects, and lead to large differences in power density absorbed, even though the incident radiation is absolutely uniform. Geometrical or chemical structures can also lead to non uniform energy absorption. For example, differing dopant levels lead to different absorption coefficients for particular wavelengths of the incident light.
These effects have been described in "Rapid Thermal Annealing--Theory and Practice", by C. Hill, S. Jones, and D Boys, NATO Summer school: Reduced Thermal Processing for ULSI, Boca Raton, Fla., 20 Jun. to 1 Jul. 1988. Other references include "Impact of Patterned Layers on Temperature Non-Uniformity During Rapid Thermal Processing for VLSI- Applications", P. Vandenabeele, K Maex, R. De Keersamaekker, 1989 Spring Meeting of the Materials Research Society, San Diego, Symposium B., Apr. 25-28, 1989; "Temperature Problems with Rapid Thermal Processing for VLSI- Applications", Dr. R. Kakoschke, Nuclear Instruments and Methods in Physics Research, B 37/38 (1989) pp. 753-759; and in Defect-Guarded Rapid Thermal Processing", Z. Nenyei, H. Walk, T. Knarr, J. Electrochem. Soc., pp. 1728-1733 140, No. 6, Jun. (1993).
Patents such as U.S. Pat. No. 4,891,499, hereby incorporated by reference, and EP 0 290 692 A1 give suggestions for making the uniformity better over the whole wafer, but do not deal with the structure induced inhomogeneities. The challenge is still the continuous improvement of the wafer to wafer temperature control for the better repeatability and the continuous improvement of the process homogeneity on the patterned frontsides of the wafers.
Japanese patent application 61-129,834 proposes quartz plates interposed between the quartz walls of the RTP chamber and the wafer. The plates transmit the short wavelength radiation from the lamps and absorb the (non-uniform) longer wavelength radiation from the hot walls of the chamber.
Japanese patent document JP A 60-211947 discloses an RTP system with an optical system arranged to keep all lamp light out of the pyrometer measuring system by absorbing it in a graphite plate.
Kanack et. al. , Appl. Phys. Lett. 55 2325 (1989) disclose a method of annealing contacts in GaInAsP by placing the InP substrate wafer between two silicon susceptor wafers.
The presence and reduction of structure induced inhomogeneities appears to be more complicated than has been presented in the literature. In fact, the reduction of structure induced inhomogeneities in RTP systems has physical limits.
In order to achieve rapid heating and high substrate temperatures, the color temperature of the lamps used in the RTP system must be much higher than the desired wafer temperature. In addition, the emitting surface area of the tungsten lamp filaments much smaller than the surface of the reflectors which distribute the light through multiple reflections in an RTP system, which requires even higher color temperature of the filaments of the lamp. A high color temperature has the disadvantage that the lamp emission spectrum is quite different from the emission spectrum radiated from the wafer. If the lamp and wafer emission spectra were the same, for example, varying absorption coefficients on the wafer would imply the same varying emission coefficients, so that if more light energy were absorbed in one place, more light energy would be radiated and the temperature would be substantially constant independent of the varying absorption constants. When the peaks of the incident energy spectra from the lamps and the radiant energy spectra from the wafer are far apart, the varying absorption constants are not balanced by identically varying emission coefficients.
In addition, identical thin film structures with different lateral dimensions will have different temperature time profiles since the thermal capacity of the neighborhood of the structure will have a larger effect for smaller area structures.
However, these "passive" structure determined inhomogeneities which are known in the literature have been neglected for small structures.
Chemical solid state reactions such as silicidization or oxidation and physical structure changes such as annealing of amorphous implanted silicon to crystal silicon are mostly exothermic processes. Some allotropic processes are, however, endothermic. These sources and sinks of energy are highly localized, and can lead to temperature inhomogeneities. We have, however, found nothing in the RTP literature that deals with such effects.
In conventional thermal processing, where the conductive or convective energy transfer is dominant and the heating rates of 1-10 C / minute are common, such structure induced temperature inhomogeneities are not important, because there is enough time for thermal conduction to "even out" such inhomogeneities. At heating rates of 10-1000 C/sec, however, such inhomogeneities can be important. In the usual RTP reactor, the wafer is heated at from 10-100 C/sec. When the desired temperature is reached, the temperature is usually held constant (steady state temperature) for a determined time. Multi step temperature time profiles can also be programmed into a modem system. Very great care is taken to test the wafer temperature homogeneity during this entire heating process, generally using unstructured wafers. Less care is taken to measure or take into account the structure generated inhomogeneities, and less effort is generally given to reducing them.
The "thermal reactions" often have their greatest reaction rate at the beginning of the "steady state" temperature period, and are usually finished in the next 10-100 seconds. Titanium and Cobalt silicide processes are a good example of such processes. Local temperature differences can be amplified under such conditions due to the rapid change of the optical properties of the new phase. In this case, such solid state reactions can proceed a different rates for different areas of material, and possibly for the smallest structures the reactions will not proceed to the desired point.
Most RTP machines have a thin rectangular quartz reaction chamber having one end open as sketched in FIG. 1. Chambers meant for vacuum use often have a flattened oval cross section. Chambers could even be made in the form of a flat cylindrical pancake. In general, the chambers are used so that the thin objects to be heated are held horizontally, but they could also be held vertical or in any convenient orientation. The reactor chamber is usually thin to bring the lamps close to the object to be heated. The reactor chamber is opened and closed at one end with a pneumatically operated door when the wafer handling system is in operation. The door is usually made of stainless steel, and may have a quartz plate attached to the inside. The process gas is introduced into the chamber on the side opposite the door and exhausted on the door side. The process gas flow is controlled by computer controlled valves connected to various manifolds in a manner well known in the art.
Reactors based on this principle often have the entire cross section of one end of the reactor chamber open during the wafer handling process. This construction has been established because the various wafer holders, guard rings, and gas distribution plates, which have significantly greater dimensions and may be thicker than the wafers, must also be introduced into the chamber and must be easily and quickly changed when the process is changed or when different wafer sizes, for example, are used. The reaction chamber dimensions are designed with these ancillary pieces in mind. Copending patent application Ser. No. 08/387,220, hereby incorporated by reference, teaches the use of an aperture in the door to regulate gas flow and control impurities in the process chamber.
The wafer to be heated in a conventional RTP system typically rests on a plurality of quartz pins which hold the wafer accurately parallel to the reflector walls of the system. Prior art systems have rested the wafer on an instrumented susceptor, typically a uniform silicon wafer.